net: lapbether: replace comparison to NULL with "lapbeth_get_x25_dev"

According to the chackpatch.pl, comparison to NULL could
be written "lapbeth_get_x25_dev".

Signed-off-by: Peng Li <lipeng321@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>